1. Field of the Invention
The present invention relates to an optical transmission system using an optical fiber cable as a transmission line.
2. Description of the Related Art
Optical transmission systems have been developed for large transmission capacity and long span transmission. For accomplishing the large transmission capacity, the increase of bit rate and wavelength division multiplexing system have been studied. For accomplishing the long span transmission, optical amplifiers are used. The optical amplifiers are categorized as for example a post-amplifier that raises the transmission power, a pre-amplifier that raises the sensitivity of received power, and an in-line amplifier that functions as a repeater. These optical amplifiers have been developed as products. With the optical amplifiers, the difference of levels of a received signal and a transmitted signal becomes large and the allowable loss of an optical fiber cable becomes large.
On the other hand, with optical amplifiers, the optical input level to the optical fiber cable becomes high. Thus, a new problem called non-linear effect has taken place. As an example, when the level of an optical signal that is input to the optical fiber cable is large (for example, +8 dBm for a dispersion shifted optical fiber cable and +10 dBm or more for a single mode optical fiber cable), a frequency (wavelength) shift takes place at a leading edge and a trailing edge of a pulse of the optical signal due to the optical Kerr effect (the refractive index varies depending on the intensity of light) (this phenomenon is referred to as self phase modulation). In this case, even if the spectral width of an optical signal before the transmission is narrow, the spectral width becomes wide through transmission. In addition, due to the influence of the dispersion of the optical signal on the transmission line, the waveform of the received signal becomes degraded. In other words, the upper limit of the power of transmission optical signal depends on such an influence.
Moreover, since the velocity of light that propagates in an optical fiber cable depends on the wavelength thereof, after an optical pulse with a particular wavelength is transmitted through an optical fiber cable, the pulse width may be expanded or compressed. This phenomenon is referred to as chromatic dispersion in optical fiber cable. Thus, after an optical signal is transmitted through an optical fiber cable of an optical transmission system, the waveform of the received signal varies due to the chromatic dispersion. Depending on the degree of the chromatic dispersion, a transmission error takes place. Thus, the transmission distance may be restricted due to the chromatic dispersion.
So far, transmission deterioration due to the chromatic dispersion on the optical fiber cable is prevented using a light source with a narrow wavelength width. However, in recent years, due to high bit rate of 10 Gb/s and the non-linear effect of an optical fiber cable, the transmission deterioration cannot be prevented using a light source with a narrow wavelength width.
To solve such a problem, an optical transmission system with a dispersion compensation has been used. However, since the cost of the dispersion compensator is high and the dispersion compensation amount varies corresponding to the transmission distance, there need be a variety of products. Thus, it is difficult to use the optical transmission system with a dispersion compensator.
In such a conventional technology, as a pre-chirp of the transmitter, a blue chirp (chirping parameter xcex1 less than 0) is used. In addition, a dispersion compensator is placed on the receiver side (between a pre-amplifier and an optical-electrical signal converter (O/E)) (namely, post compensation is performed). However, in this system, since the compensation is inflexibly performed, the loss of the dispersion compensator becomes large. The loss cannot be ignored when the transmission distance becomes long. In addition, since the input level of the optical signal becomes low, the receiver sensitivity degrades. Moreover, since the tolerance of the dispersion compensation amount for proper transmission characteristics is narrow, dispersion compensators should be prepared corresponding to the transmission distance. Thus, many types of products should be prepared. To solve such a problem, a system in which a red chirp (chirping parameter xcex1 greater than 0) is used as a pre-chirp on the transmitter side and dispersion compensators are disposed on both the transmitter side and the receiver side was considered. FIG. 1 shows a basic structure of this system.
FIG. 1 is a block diagram showing an outlined structure of a conventional optical transmission system.
The optical transmission system shown in FIG. 1 comprises a transmitter 160, a transmission line 164 (composed of an optical fiber cable), and a receiver 165. The transmitter 160 comprises an E/O (electric-optical signal converter) 161, a dispersion compensator 162, and a post-amplifier 163. The E/O 161 converts an electric signal into an NRZ coded optical signal. The post-amplifier 163 amplifies the optical signal and sends the resultant signal to the transmission line 164. The receiver 165 comprises a pre-amplifier 166, a dispersion compensator 167, and an O/E (optical-electric signal converter) 168. The pre-amplifier 166 amplifies weakened light that has been transmitted through the transmission line 164. The dispersion compensator 167 compensates for the dispersion of the optical signal that has been transmitted through the transmission line 164. The O/E 168 converts an optical signal into an electric signal.
In the conventional optical transmission system, the transmitter 160 red-chirps an optical signal as a pre-chirp. In addition, the transmitter 160 uses an NRZ coded signal as an optical signal. The dispersion compensator 162 in the transmitter 160 compensates for a predetermined dispersion amount of an optical signal so as to cancel the dispersion of the optical signal propagated on the transmission line 164. The post-amplifier 163 amplifies the intensity of an optical signal so that it can be transmitted for a long distance.
The pre-amplifier 166 in the receiver 165 amplifies a weakened optical signal propagated on the transmission line 164 so that the optical signal can be detected. The dispersion compensator 167 adjusts the dispersion compensation amount corresponding to a dispersion amount of the transmission line 164 detected by the receiver 165 so that the receiver 165 can correctly detect the optical signal. Thus, the dispersion compensator 167 in the receiver 165 can adjust the dispersion compensation amount. The O/E 168 converts an optical signal into an electric signal. The O/E 168 sends the received signal to an electric signal processing unit (not shown) disposed downstream thereof so as to demodulate the electric signal and extract data from the optical signal.
Thus, in the system shown in FIG. 1, the transmitter 160 red-chirps an optical signal. In addition, both the transmitter 160 and the receiver 165 have respective dispersion compensators.
In this compensation system, the dispersion compensation on the transmitter side is effective. The transmitter compresses pulses corresponding to the chirping and the characteristics of the dispersion compensator. Thus, inter-symbol interference due to the increase of the pulse width on the transmission line is alleviated. In addition, since the red-chirping is used, the influence of the non-linear effect (SPM) on the transmission line is canceled. Thus, the deterioration of the waveform of the transmission signal is smaller than that in the case of the blue-chirping. Thus, since the tolerance of the compensation amount is wide, the number of types of dispersion compensators can be reduced.
However, as a problem of the system, since the dispersion compensation amount is large, many dispersion compensation optical fiber cables that are expensive should be used. Thus, the cost of the system becomes high. In addition, since the transmitter and the receiver require respective dispersion compensators, the size of the system becomes large.
Therefore, an object of the present invention is to provide a technology for accomplishing an apparatus that compensates both the chromatic dispersion of an optical fiber cable and the transmission deterioration of an optical signal due to the non-linear effect, that has sufficient transmission characteristics for a high-power and long-distance optical transmission system, and that is structured at lower cost and in a smaller size than before.
An optical transmission system according to the present invention comprises a transmitter that transmits an optical signal, a transmission line that propagates the optical signal, and a receiver that receives the optical signal transmitted through the transmission line. In a high-output-power and high-transmission-rate optical transmission system of which an optical signal propagated in the transmission line is dispersed due to the non-linear effect, the transmitter generates an RZ coded optical signal and transmits the resultant signal.
Conventionally, such an optical transmission system with high output power and high transmission rate uses an NRZ coded signal. However, since such a signal is subject to inter-symbol interference, the dispersion compensation amount is large. A dispersion compensator that compensates the dispersion of an optical signal is expensive. In addition, the dispersion compensator requires a space to be disposed. Thus, such a dispersion compensator prevents the cost and size of the apparatus from being reduced.
In contrast, according to the present invention, since an RZ coded signal is used, the inter-symbol interference can be suppressed. Thus, the dispersion compensation amount can be reduced. Alternatively, according to the present invention, an optical signal can be transmitted for a longer distance with the same dispersion compensation amount. Thus, the cost and size of the apparatus can be reduced.
In addition, although the receiver has a dispersion compensator that compensates the dispersion of an optical signal propagated through the transmission line, the transmitter does not require a dispersion compensator. Thus, the present invention contributes to reducing the size of the transmitter.
Moreover, since the transmitter pre-chirps an RZ coded optical signal and transmits the resultant signal, the optical signal can be prevented from being affected by the non-linear effect (that is proportional to the output power of the optical signal) on the transmission line. Thus, the optical signal can be transmitted for a long distance with a small dispersion compensation amount.
In the optical transmission system according to the present invention, the dispersion compensator disposed on the receiver side is composed of a plurality of dispersion compensation units each of which has a predetermined or standardized dispersion compensation amount.
Thus, when the dispersion compensation amount is adjusted on the receiver side, with a combination of the dispersion compensation units, it is not necessary to prepare a dispersion compensator corresponding to a required dispersion compensation amount. Consequently, according to the present invention, the optical transmission system can compensate for the dispersion of an optical signal simply and inexpensively.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying drawings.